1. Field
The present invention relates to a circuit board for mounting a semiconductor circuit component having a surface mount type package. Particularly it relates to a circuit board for mounting a surface mount type circuit component which is formed so that electrodes formed in the bottom of a package are connected to pads of the board through metal bumps such as solder balls.
2. Description of the Related Art
As a wide variety of such packages serving as semiconductor circuit components to be mounted on circuit boards have advanced in recent years, surface mount type leadless packages which do not use any lead or pin for electric connection to the boards have been put into practical use broadly.
For example, a BGA (Ball Grid Array) package, an LGA (Land Grid Array) package, etc., are used as such surface mount type leadless packages. In the BGA package, connection terminals are formed in such a manner that solder balls are attached to electrodes (pads) which are formed, for example, as a grid array, on the bottom of the package so as to be connected to an external circuit. In the LGA package, pads (lands) formed as a grid array on the bottom of the package are used directly as connection terminals.
There is also known a CSP (Chip Size Package) having substantially the same size as that of a bare chip constituting a semiconductor. This CSP can be configured so that pads are provided on the bottom of the package so as to be connected to pads of a circuit board through solder balls or other metal bumps.
The BGA package is mounted on a circuit board by so-called reflow soldering which is performed in such a manner that soldering is performed by heating the solder balls while the solder balls are brought into contact with pads of the circuit board to be connected.
The LGA package is mounted on a circuit board through a socket provided with contactors being in contact with pads formed on the bottom of the package or the LGA package is mounted directly on a circuit board through solder balls attached to the pads.
The CSP can be also mounted on a circuit board substantially in the same manner as these circuit components.
When the circuit board mounted thus with the surface mount type semiconductor having no lead is attached to the inside of a housing of an electronic apparatus to use the circuit board, the circuit board is incorporated as one of constituent members of circuits in the electronic apparatus.
In a process of executing the circuit board incorporating operation, stress may be sometimes applied on the circuit board in a direction crossing a board surface or shock may be applied on the circuit board because of falling, etc. As a result, the circuit board may be warped or distorted. This may affect junction portions between the circuit component and the board, so that there is a possibility that the pads of the board will be peeled or soldered portions will be broken. There is a problem that the circuit board cannot fulfill its original function.
If the pads of the circuit board are peeled from the board body, the circuit board per se has to be exchanged for a new one. Accordingly, a considerably great deal of labor is required for assembling the electronic apparatus.
Particularly in recent years, there has been used the circuit board per se made of a thin material for reduction in size and thickness of the outer appearance of the electronic apparatus. In this case, the stiffness of the board is lowered so that the board is apt to be deformed by stress or shock applied externally. There is an increasing possibility that such an accident that peeling of the pads in the circuit board for mounting a surface mount type circuit component using no lead such as a BGA circuit component will occur.
For example, JP-A-2003-78240 has heretofore disclosed a proposal to prevent lowering of peel strength of the pads in such a manner that the pads of the circuit board mounted with a BGA circuit component or a CSP circuit component are formed so that pads in outermost circumferential corner portions become larger in diameter than pads in the other portions. In this proposal, the space of the circuit board cannot be used effectively because the pads are circular. Moreover, when external stress is applied, the external stress is concentrated at one point of an outer edge of each pad. There is therefore a problem that the pads are apt to be peeled.